This invention relates to a spool type direction control valve for controlling the flow direction of an hydraulic circuit and, more particularly, to a direction control valve fitted with a flow control mechanism.
A variety of direction control valves of this type have been used thus far in the art. Typical of these is the construction disclosed in Japanese Patent Publication No. 49-21693.
FIG. 1 shows a direction control valve of the spool translation type which is used for controlling the flow of the pressure oil into and from a tilting cylinder for a forklift truck.
In the operation of the direction control valve, when a spool member 31 is displaced towards the left in FIG. 1, a plunger 33 is shifted towards the left by the pressure prevailing in an oil supply port 32 and against the force of a spring 34 so as to establish hydraulic communication between grooves 35 and 36 so that the pressure fluid from the supply port 32 is conveyed from the supply port 32 through these grooves 35 and 36 and a connecting pipe 37 to the head side oil chamber 38a in the tilting cylinder 38. On the other hand, since the displacement of the plunger 33 puts the grooves 39 and 40 into communication with each other, the pressure fluid in the rod side oil chamber 38b to a tank port 42 through a connecting pipe 41 and the grooves 39, 40, so that the mast is tilted forwards. Conversely, when the spool member 31 is displaced towards the right, the pressure oil from the supply port 32 causes a check valve 43 to open so that it may flow from the groove 40 into the groove 39 such that the pressure oil is supplied through the connecting pipe 41 into the rod side oil chamber in the tilting cylinder 38. At this time, a small plunger 44 is shifted towards the right in FIG. 1 by the fluid pressure prevailing in the supply side, against the force of a spring 45, so as to establish hydraulic communication between the grooves 35 and 36 through a groove 46 formed in the small plunger 44 so that the pressure fluid in the head side oil chamber in the tilting cylinder 38 flows into the tank port 47 through the connecting pipe 37 and grooves 36, 35 for tilting the mast rearwards.
The above described direction control valve shown in FIG. 1 has a defect in that it is highly complicated in structure. In addition, since the sectional area of the pressure oil passage during mast tilting is dictated by the plunger position controlled in turn by the spring and the pilot pressure of the supply side pressure fluid, the speed of forward tilting during such forward mast tilting is changed as a function of the load acting in the forward tilting direction, even granting that the pressure fluid in the rod side oil chamber of the tilting cylinder is discharged via throttling means. Also, since the passage is opened and closed by plunger operation, the pressure fluid cannot but leak under the shut-off state with the spool in the neutral position. Thus there is an additional defect in that the mast, which is subject at all times to the load acting in the forward tilt direction, can not be maintained for a prolonged time at a fixed position.